It is well known that differential switches are components widely used as protection instruments in buildings and/or industrial plants (commonly called an automatic cut-out) or as instruments for verifying the correct use of electric supplies (for example, as anti-tampers in electronic meters).
In particular, a differential switch is connected to the cables conducting the electric current, usually indicated as phase and neutral. In general, the current enters through the phase, crosses the circuits of the system connected to the differential switch, and goes out from the neutral.
Under normal conditions the entering current must be equal to the current going out. If this does not occur, it means that a part of the current is crossing different paths, such as a human body in the case of an electric shock (direct contact) or dispersion paths due to insulator failure, for example, of a household appliance connected to a ground system.
The differential switch thus continuously compares the entering current with the current going out, and intervenes when it detects a difference of current, indicating an “alarm” event.
At present, there exist two main categories of differential switches:
1) electromechanical differential switches that are based on a mechanical system for the activation of the switch in the presence of the alarm event; and
2) electronic differential switches which use an electronic circuit for the detection of the alarm event and the activation of the switch.
Especially when used as protection instruments, human life being possibly at stake, these differential switches require particular safety guarantees which minimize the likelihood of missed intervention due to a possible failure of the switch itself.
All the differential switches on sale also have a “test” button (usually indicated with a letter T stamped on it) for verifying the functionality of the switch itself by generating a leakage current inside the circuit that forms the switch.
A differential switch of the electromechanical type that is commercially available is shown in FIG. 1.
In particular, the electromechanical differential switch 1 comprises a toroid 2 in which the cables of an electric network are inserted, in particular a phase cable L1 and a neutral cable L2.
The toroid 2 is associated with a first winding W1 that is connected, through a first decoupling capacitor Cd1, to a second winding W2, which is associated with a relay 3, which is connected to a driving mechanical system 4 for a pair of switches Sw1 and Sw2 that are connected to the phase L1 and neutral L2 cables, respectively. The switches Sw1 and Sw2 form the main switch T. In substance, the first winding W1 is the winding measuring the differential current while the second winding W2 is the winding exciting the relay 3.
The electromechanical differential switch 1 also comprises a pair of diodes D1 and D2—in opposite configuration with respect to one another—and the first decoupling capacitor Cd1, inserted in parallel to the windings W1 and W2, as well as a second decoupling capacitor Cd2, inserted in series between the pair of diodes D1 and D2 and the first decoupling capacitor Cd1.
The electromechanical differential switch 1 further comprises a test circuit 5 that is inserted between the phase cable L1 and the neutral cable L2 and includes the series of a resistance 6 and a test switch Sw3 associated with a test button of the differential switch, shown by a test driving signal Test.
An electronic differential switch of the type shown in FIG. 2 is also known. Elements being structurally and/or functionally identical to the electromechanical differential switch 1 described with reference to FIG. 1 are given the same reference numbers for sake of simplicity.
The electronic differential switch 10 comprises a toroid 2 in which a phase cable L1 and a neutral cable L2 of an electric network are inserted.
The toroid 2 is associated with a first winding W1, which is connected to an integrated circuit 8, which is connected to a diode 9 SCR, which drives a second winding W2, which is associated with a relay 3. The relay 3 directly drives a pair of switches Sw1 and Sw2 that are connected to the phase L1 and neutral L2 cables, respectively. The switches Sw1 and Sw2 form the main switch T.
In this cases the second winding W2 has a first end directly connected to the neutral cable L2 and a second end connected to the phase cable L1 through a diode bridge 7.
In particular, the diode bridge 7 has respective first N, second E, third S, and fourth W terminals, with the first terminal N being connected to the second end of the second winding W2 and the third terminal S being connected to the phase cable L1.
The electronic differential switch 10 also comprises the integrated circuit 8, for measuring a differential current, inserted between the first winding W1 and the diode 9 SCR (“Silicon Controller Rectifier”).
In particular, the integrated circuit 8 has a first input terminal IN1 directly connected to a first end of the first winding W1 and a second input terminal IN2 connected to a second end of the first winding W1 through a first capacitor C1, as well as a first output terminal O1 connected to a first end of the diode 9 SCR through a first resistor R1, a second output terminal O2 feedback connected to the second input terminal IN2 through a second resistor R2, and a third output terminal O3 connected to a driving terminal of the diode 9 SCR.
The integrated circuit 8 also has a first biasing terminal T1 connected to the second terminal E of the diode bridge 7 through a second capacitor C2 for timing the system intervention, and a second biasing terminal T2 that is also connected to the second terminal E of the diode bridge 7. The second terminal E is connected to a ground voltage reference GND.
The diode 9 SCR has a first end connected to the fourth terminal W of the diode bridge 7 (and thus to the first output terminal O1 of the integrated circuit 8 through the first resistor R1) and a second end connected to the second terminal E of the diode bridge 7 (and thus to the first biasing terminal T1 of the integrated circuit 8 through the second capacitor C2 as well as to the second biasing terminal T2 of the integrated circuit 8).
The differential switch 10 further comprises a test circuit 5 that is inserted between the phase cable L1 and the neutral cable L2 and includes the series of a resistance 6 and a test switch Sw3 associated with a test button of the differential switch, shown by a test driving signal Test. The value of the resistance 6 determines the simulated failure current.
The integrated circuit 8 measures a differential current correlated to the currents IF and IN flowing in the phase L1 and neutral L2 cables, respectively.
In particular, if the current IF circulating in the phase cable L1 is equal to the current IN flowing back through the neutral cable L2 (i.e., if the condition IF−IN=0 occurs), there is no linked flow in the toroid 2 inducing a void voltage in the first winding W1, and consequently the second winding W2 will not be excited by the diode 9 SCR.
Under these conditions, the integrated circuit 8, detecting no voltage difference at its input terminals IN1 and IN2 (which are connected to the first winding W1), maintains the diode 9 SCR off through a suitable voltage value applied thereto by the third output terminal O3. In this way the relay 3 is not excited and the main switch T (connected to the diode 9 SCR through the diode bridge 7) remains closed.
If, for any reason, a current leakage to ground occurs downstream of the electronic differential switch 10, the difference between the two currents IF and IN circulating in the phase L1 and neutral L2 cables is different than zero with consequent presence of a linked flow in the toroid 2, and thus non-void voltage variation at the terminals of the first winding W1. This voltage variation is applied at the input terminals IN1 and IN2 of the integrated circuit 8, which, after a fixed time determined by the second capacitor C2, activates the diode 9 SCR through its third output terminal O3 with consequent excitation of the relay 3 and opening of the main switch T.
This condition remains until the electronic differential switch 10 is reset from the outside.
Although advantageous under several aspects, this first solution has several drawbacks including the following.    1) It does not ensure the intervention of the electronic differential switch 10. In fact, the only way to verify the operation of the switch itself is to activate the “test” button and thus the test circuit 5. Ensuring that the switch does not fail after the execution of this test operation is not possible; electromechanical differential switches also have this drawback.    2) In case of accidental intervention of the electronic differential switch 10 due to a sudden alarm event (such as a lightning strike), the protected condition of the main switch release remains after the end of the alarm event. In particular, the known type of electronic differential switch 10 is not able, unless it uses more complex systems, to distinguish between a real failure current and one due to an external disturbance or noise which is introduced into the controlled system, with consequent problems in case of protections inserted in the presence of electric apparatuses requiring a continuous supply.